The present invention relates to a light power controlling apparatus for automatically controlling light power generated by a semiconductor light emitter, e.g. a semiconductor laser. The output characteristics of a semiconductor light emitter such as a semiconductor laser describes that a forward current more than a threshold voltage is required for the light emission of the semiconductor light emitter. The threshold value is a function of temperature, and increases with temperature rise. To keep the light power at a fixed value, it is necessary to regulate the forward current fed to the semiconductor light emitter. For this regulation, the light power controlling apparatus is used. In the conventional light power controlling apparatus, the semiconductor light emitter is connected through a switching circuit for modulation to a current regulator. A photosensor, receiving part of the light from the semiconductor light emitter, produces an electrical signal for transfer to a current-voltage converter, i.e. an I-V converter. The output voltage of the I-V converter, i.e. a detecting voltage, is compared, by an error amplifier, with a reference voltage representative of a desired value of the light power. The error amplifier produces an output signal representing a difference between the detected value and the desired value. The difference signal is integrated by an integrator. The integrated signal from the integrator is supplied, as a feedback control signal, to the current regulator. Upon receipt of this signal, the current regulator supplies forward current to the semiconductor light emitter. In this way, the output light power of the semiconductor light emitter is kept at a fixed value.
The above controlling apparatus operates so as to zero the difference between the average value of the light power and the desired value. No problem in particular arises when the semiconductor light emitter operates in a continuous light emission mode. The following problem, however, arises when the light output of the semiconductor light emitter is modulated. When the switching duty ratio of the switching circuit changes, the peak value of the modulated light also changes. For 40:10 of the duty ratio of the modulated waveform, an average value of the modulated light is approximate to its maximum value. For 10:40 of the duty ratio, the average value of the modulated light is much smaller than the average value in the case of the former duty ratio. This implies that the light power changes with the duty ratio.
For a high switching speed, the feedback system can not follow the high switching speed at the start and end of the operation. The result is a great fluctuation of the output power. This problem may be improved by increasing the response speed of the feedback system up to a value satisfactorily higher than the switching cycle. This measure still involves problems, however. A high frequency signal of several MHz of more is treated in the optical disk or the laser printer. Such device is inevitably accompanied by stray capacitance. Therefore, this necessitates some means to remove the stray capacitance, leading to restriction in variation of usable circuit components and high cost to manufacture.